1. Field of the Invention
The present invention relates to duplex printing transfer paper and an electrophotographic recording method used in indirect dry electrophotographic full-color or monochrome copying machines and printers.
2. Description of the Related Art
Along with the development of color copying machines and printers as well as digitalization of these systems, high definition of an electrophotographic copying machine and printers have been investigated. In particular, digitalization of input/output information has advanced for obtaining a high quality image with a full-color electrophotographic copying machine or printer and brought about great improvements in image input, image processing, development, transfer, fixing, and the like. Developers and photoreceptors have also been improved in conformity with the tendencies of digitalization, high definition, and high color development recording.
First, a color image forming method will be described below.
FIG. 1 is a general structural diagram of a color image forming apparatus, which comprises an automatic document supply unit 1, an image input portion 2, an image output portion 3, and a paper supply portion 4. Copy cycle in the case of a full color mode will be described below. A color document is set on platen glass 5 by the automatic document supply unit 1. The image input portion 2 includes an imaging unit 6, and a wire 7, a drive pulley 9, etc. for driving the unit. In the case of four colors in full, the image input portion 2 reads the color document by B (blue), G (green) and R (red) as primary colors of light and converts the color document into a digital image signal by using a CCD line sensor and a color filter disposed in the imaging unit 6. Then, the image input portion 2 converts this signal into Y (yellow), C (cyan), M (magenta) and K (black) as primary colors of toner and further converts the color gradation toner signal into an on/off two-valued signal by applying various data processing in order to improve reproducibility in color, gradation, definition, etc. Thus, the image input portion 2 outputs the two-valued signal to the image output portion 3.
The image output portion 3 includes a scanner 10, and a photosensitive material drum 11. Further, there are arranged an electrifier 12 for electrifying the photosensitive material drum 11 uniformly, a developer unit 13 for developing an electrostatic latent image to a toner image, a transfer drum 16 for transferring the toner image onto paper, and a cleaner 15 for recovering the residual toner which has not transferred. The photosensitive material drum 11 is driven by an electric motor so as to rotate in the direction of the arrow shown in the drawing.
In a laser output portion 10a of the scanner 10, for example, a yellow image signal from the aforementioned image input portion 2 is converted into a light signal so that a latent image corresponding to the document image is formed on the photosensitive material drum 11 through a polygon mirror 10b, an f/.theta. lens 10c and a reflection lens 10d. If this yellow latent image is transferred onto paper through development, then the residual toner is removed from the photosensitive material drum 11 by the cleaner 15 and then the photosensitive material drum 11 is electrified by the electrifier 12 so that the laser output portion 10a outputs a cyan image signal. Thereafter, latent images of magenta and black image signals are formed successively.
The developer unit 13 has a yellow developer 13Y, a cyan developer 13C, a magenta developer 13M, and a black developer 13K. The respective developers are arranged in the periphery of a rotary shaft. When, for example, a yellow toner image is to be formed, development is performed by the yellow developer 13Y in the position shown in the drawing. When, for example, a cyan toner image is to be formed, the development unit is rotated so that the cyan developer 13C is arranged in a position where the cyan developer 13C touches the photosensitive material drum 11. Magenta and black developments are carried out in the same manner as described above.
A dielectric film or a mesh screen is put up in the outer circumference of the transfer drum 16. The transfer drum 16 is connected to an exclusive-use electric motor or the photosensitive material drum 11 by a gear so that the transfer drum 16 is driven to rotate in the direction of the arrow shown in the drawing. A transfer electrifier 17, a separation electrifier 19, a peel claw 20, a destaticizer 21, a cleaner 22, a push roll 23 and an adsorption electrifier 25 are arranged in the periphery of the transfer drum 16. Transfer paper carried from the paper supply portion 4 via paper supply rollers 26 and paper supply guides 27 is held on the dielectric film or mesh screen by corona of the adsorption electrifier 25. The transfer drum 16 rotates in synchronism with the photosensitive material drum 11, so that, for example, a toner image developed by yellow is transferred onto the paper by the transfer electrifier 17 and other colors are transferred successively by the rotation of the transfer drum 16.
When transfer of four colors is completed by four turns of the transfer drum, the transfer drum 16 is AC-destaticized by the separation electrifier 19 provided on the transfer drum 16, so that the paper is separated by the peel claw 20 and fed to a fixer 30 by a carrying belt 29. The toner image is melted and fixed by hot-press rollers 31. Thus, a copying cycle is completed. In the case where images are to be formed on opposite surfaces of transfer paper, the aforementioned copy cycle is repeated so that an image can be formed on a surface opposite to the surface of transfer paper on which an image has been fixed and formed.
Further, a method in which a toner image on a photosensitive material drum is once primarily transferred onto an intermediate transfer material other than transfer paper and then the toner image is secondarily transferred onto the transfer paper to thereby obtain a copy image, has been described in Unexamined Japanese Patent Publication No. Sho-62-206567. FIG. 2 is a diagram showing a copying machine using such an intermediate transfer material. The reference numeral 100 designates a photosensitive material drum. On the surface of the photosensitive material drum 100, electrophotographic process executing means such as a primary electrifier, an image exposure means, a developer, etc. not shown are provided so that a toner image T is formed. The toner image T formed on the surface of the photosensitive material drum 100 is fed to a primary transfer position with the rotating operation of the photosensitive material drum 100.
The reference numeral 101 designates an endless belt-like intermediate transfer material laid between a plurality of rollers. The intermediate transfer material 101 in the primary transfer position is arranged so as to be in contact with or near the surface of the photosensitive material drum 100. The reference numeral 102 designates a primary transfer corona discharger disposed on the back side with respect to the primary transfer position of the intermediate transfer material 101. A voltage having polarity reversed to the toner charge polarity on the photosensitive material drum 100 is applied to the corona discharger 102 to thereby perform electric discharge. The reference numeral 103 designates a secondary transfer bias roll for holding transfer paper 104 fed from a paper supply tray 105, between the roll 103 and the intermediate transfer material 101. A transfer voltage having polarity reversed to the toner charge polarity is applied to the secondary transfer bias roll 103. The reference numeral 106 designates a feed roller for feeding the transfer paper 104 placed on the paper supply tray 105 toward the intermediate transfer material 101; 107, a peeling claw having an end freely touching the intermediate transfer material 101 in a peeling position; and 108, a carrying belt for carrying transfer paper peeled by the peeling claw 107 toward a fixing unit not shown.
The transfer paper on which the toner image has been transferred is fed from the carrying belt to a hot-press roller fixing unit not shown. In the hot-press roller fixing unit, the toner image is melted and fixed, so that a copy cycle is completed. In the case where images are to be formed on opposite surfaces of transfer paper, a toner image is secondarily transferred from the intermediate transfer material 101 onto a surface opposite to the surface of transfer paper on which the aforementioned image has been fixed and formed, and then the toner image is melted and fixed by the hot-press roller fixing unit to thereby make it possible to form images on opposite surfaces of transfer paper.
In the aforementioned color image forming method, an attempt to generate images on opposite surfaces of transfer paper has been made. For example, a method in which color duplex transfer paper with the opacity of not lower than 90% and with the brightness of not lower than 85% is used for forming color images on opposite surfaces of the transfer paper, has been proposed in Unexamined Japanese Patent Publication No. Hei-6-186769.
As transfer paper for forming color images in the electrophotographic system, transfer paper formed by coating high-quality paper with a small amount of coating composition in order to attain stability of copy curling, high whitening for improvement of color development and higher image quality -has been proposed in Unexamined Japanese Patent Publication Nos. Hei-4-268567, Hei-4-291351, Hei-4-337736, Hei-4-349468, Hei-5-53363, Hei-5-341553, etc.
Further, as transfer paper having specifications of coating paper to obtain high-gloss images, various proposals for improvement mainly in runnability and toner image fixing characteristic of transfer paper have been made in Unexamined Japanese Patent Publication Nos. Sho-62-198875 to 198877, Hei-1-57276, Hei-3-294600, Hei-5-19522, Hei-5-216322, Hei-6-19178, etc.
It has however become clear that serious image failure occurs particularly under a low-humidity environment when images are formed on opposite surfaces of such transfer paper proposed for forming color images or conventional transfer paper used in monochrome electrophotographic copying machines by using the full-color image forming apparatus as shown in FIGS. 1 and 2.
That is, when duplex images are formed under a low-humidity environment, white or thin spots in a diameter range of from about 1 mm to about 2 mm or bird-claw-like white or thin decolored portions in a range of from about 1 mm to several mm are formed in an image portion generated on the second surface. This phenomenon does not occur in the case where duplex copies are generated by using a conventional monochrome copying machine or in the case where a simplex image is formed by using a full-color image forming apparatus as shown in FIGS. 1 and 2. This phenomenon is peculiar to the case where duplex images are formed under a low-humidity environment by using such a full-color image forming apparatus. This phenomenon has been seen from conventional knowledge.
The inventors of the present invention have examined this phenomenon carefully. As a result, this phenomenon does not occur at the time of fixing the second-surface image but occurs after the second-surface image is transferred. Examining more in detail, this phenomenon is caused by the fact that toner images which have not been fixed on transfer paper are made to scatter by a phenomenon of electric discharge from transfer paper in the second-surface transfer region. There has been required transfer paper free from image failure caused by electric discharge which occurs when full-color duplex images are formed under a low-humidity environment. Besides this requirement, requirements for full-color duplex recording transfer paper are nonshowthrough, suppression of curling, smooth and sharp image quality, high color development, excellent paper feeding characteristic, etc.
As transfer paper for forming full-color duplex images, a proposal in which proportion of fillers to be mixed is improved to attain high opacity to thereby provide nonshowthrough has been made in Unexamined Japanese Patent Publication No. Hei-6-186769; and a proposal in which a release agent from fixing rolls is absorbed to transfer paper to thereby prevent image stain on the second surface and a proposal for transfer paper having a release agent absorbing layer therefor have been made in Unexamined Japanese Patent Publication No. Hei-5-127547.
In any case, there is however no proposal for improving image failure caused by electric discharge which occurs when full-color duplex images are formed under a low-humidity environment.
Image failure caused by electric discharge which occurs when full-color duplex images are formed under a low-humidity environment will be explained below with reference to FIG. 1. In full-color copying, multicolor toner (generally, toner of four colors consisting of yellow, magenta, cyan, and black) formed on transfer paper are melted and mixed with each other sufficiently by hot pressure of a fixing unit designated by the reference numeral 30 to thereby obtain sharp color development. Therefore, thermal capacity given to transfer paper by the fixing unit is large compared with the conventional monochrome copying machine using black toner.
Accordingly, the water content of transfer paper in this occasion is small compared with the case where the monochrome copying machine is used. When a copy is to be made onto the back surface, this transfer paper is placed on the paper supply portion 4 again and then development images of respective colors on the photosensitive material drum 11 are electrostatically transferred, by corona discharge for each color by means of the transfer electrifier 17, onto this transfer paper carried by the transfer drum 16. On the transfer paper which has once passed through the fixing unit under a low-humidity environment, electric charges received at the time of transferring toner of respective colors are accumulated so that an electric discharge phenomenon occurs in a portion where the transfer drum 16 and the photosensitive material drum 11 are adjacent to each other.
Further, in the case of a transfer method using an intermediate material as shown in FIG. 2, toner of four colors (yellow, magenta, cyan and black) at maximum must be transferred collectively by applying a transfer voltage having polarity reversed to the toner charge polarity by using the secondary transfer bias roll 103. Accordingly, the transfer voltage in one transferring operation is high compared with the case where the monochrome copying machine is used or the case where toner of respective colors as shown in FIG. 1 is multiplex-transferred onto transfer paper.
Accordingly, also in the transfer method using such an intermediate material as shown in FIG. 2, in the case of specifications in which an image is formed on the back surface of transfer paper once having passed through the fixing unit particularly under a low-humidity environment, a phenomenon of electric discharge from transfer paper occurs in a region near the secondary transfer bias roll 103 holding transfer paper between the bias roll 103 and the intermediate material 101 and supplied with the transfer voltage having polarity reversed to the toner charge polarity. Further, the same image failure caused by the electric discharge phenomenon occurs in a method of multiplex-transferring development images of respective colors from the photosensitive material or intermediate material, or in a method of collectively transferring development images of multicolors from the photosensitive material or intermediate material, as well as the method shown in FIGS. 1 and 2.